Generally, when conducting a call or handling a connection from a terminal such as a mobile terminal or phone, a combinational service can be of advantage. As an example, such a combinational service can be created by adding, to a circuit-switched, CS, call, one or more packet-based sessions such as at least one IMS session, between the same participants, or vice versa. IMS stands for Internet Protocol, IP, multimedia subsystem. CS & IMS combinational services, CSICS, are specified in Draft 3GPP Rel-7 specifications TS 22.279V7.1.0 (2005-12): Combining CS and IMS sessions; Stage 2, and Stage 3, TS 22.279V1.0.0 (2006-02).
Such a combinational service allows a CSICS capable mobile or stationary terminal such as a user equipment, UE, to have simultaneous peer-to-peer CS and IMS communications which can be presented within one context to the user of the terminal. To achieve this, the communicating UEs shall indicate their CSICS capabilities to each other and be aware of the capabilities of the radio access network, i.e. whether the network supports simultaneous CS and packet-switched, PS, services. A capability exchange procedure, based on the exchange of SIP messages, can be applied between the communicating UEs.
A CS multimedia service with service change and fallback, SCUDIF, Service Change and UDI/RDI Fallback is known and described in 3GPP TS 23.172 V6.3.0 (2005-06): UDI/RDI fallback and service modification, Stage 2. UDI stands for unrestricted digital information, RDI for restricted digital information. UDI/RDI is part of Bearer Capability information passed to/from the network. An “information transfer capability” can be set to either UDI or RDI. The service change within SCUDIF means the capability of a user to change from e.g. a video call, or more generally a multimedia call including rich content such as visual or acoustical non-speech content, to a speech call and vice versa during an ongoing call. In SCUDIF, the change is signalled through the network to the far end terminal and requires bearer independent call control, BICC, signalling in the core network. However, such a service change does not work with Integrated Services Digital Network, ISDN, User Part, ISUP, or less developed signalling systems. Further, a problem with SCUDIF is that it requires BICC support throughout the core network. Enhancing ISUP to support SCUDIF may be considered but can be problematic.
When a CS video call is established between CSICS capable UEs, standard CS video call setup routines, including a slow inband codec negotiation, can be performed. It is desirable to speed up the establishment of a call or connection of this or other types.